Transmission Line

Introduction

A transmission line (TL) is simply a medium that is capable of guiding or propagating electromagnetic energy. The transmission line store the electric (E) and magnetic (M) energies and distribute them in space by alternating them between the two forms. This means that at any point along a TL, energy is stored in a mixture of E and M forms and, for an alternating signal at any point on the TL, converted from one form to the other as time progresses (Steer, 2009). Transmission line is usually modelled using lumped elements, i.e. inductors for magnetic energy, capacitors for electric energy, and resistors for modelling losses. According to Collin (2000), the electrical characteristics of a TL such as the propagation constant, the attenuation constant, the characteristic impedance, and the distributed circuit parameters can only be determined from the knowledge of the fields surrounding the transmission line.

Two broad categories of transmission lines are planar and non-planar lines. Popular planar TLs include slotline, stripline, coplanar waveguide and microstrip line; while non-planar transmission lines are mostly co-axial and waveguides. Planar transmission lines have their entire conducting metal strip lying on a plane. Hence, planar circuit components and devices are designed on the two dimensional (2D) layout, as the engineer/designer has no control over the depth/thickness of the substrate which are normally predetermined. A common way of constructing a planar transmission line is by placing one or more parallel metal strips on a dielectric substrate material, adjacent to a conducting ground plane (Collin, 2000). Non-planar transmission lines such as waveguides allow 3D design control. This means that the thickness/depth of materials can be altered and hence, act as one of the design parameters. Waveguide transmission lines are three dimensional (3D) and have one conductor (Steer, 2009). It has been reported that early microwave circuits, components, devices and systems relied on waveguide and coaxial TLs for electromagnetic (EM) wave propagation (Pozar, 2004). Devices constructed based on waveguide transmission line are well known for their high quality factor, low loss and better power handling capabilities. However, they are bulky (especially at lower frequencies) and expensive to construct. Coaxial transmission line has very high bandwidth and is very convenient for test applications, however, it is very difficult to fabricate or manufacture for complex microwave components (Pozar, 2004). Planar transmission line devices, on the other hand, are well known for their compact size, low cost, ease of manufacturing, and ease of integration with active devices such as diodes and/or transistors to form microwave integrated circuits (Pozar, 2004). The down side to planar TL devices is that they suffer from low power handling capabilities (Packiaraj et al., 2005) and are more susceptible to losses when compared to waveguide devices.

Chen and Wu (2014) gave a detailed analysis of the SIW which is a twenty-first century transmission line that has evolved to bridge the gap between planar and waveguide transmission lines. This new type of TL has changed the paradigm relating to the development of circuits, components, devices, sub-systems and systems operating in the microwave and millimetre-wave frequency range. The SIW is actually a planar structure but have a waveguide like performance. This means that the SIW combines the merits of microstrip, i.e. compact size, low cost, easy to manufacture and easy integration with active devices; with the merits of waveguides, i.e. low radiation loss, high unloaded quality factor, and high power handling capabilities. Cheng (2015) described the SIW as a dielectric-filled waveguide that is synthesized by two rows of metalized vias, embedded in a dielectric substrate with conductor claddings on the top and bottom walls. Some other main merits of the SIW, besides its high quality factor and high power handling capability, include: its ability to integrate typical waveguide components in planar form, the flexibility of its design, and the comprehensive shielding of its structure. SIW devices can be manufactured or fabricated using different technologies including the printed circuit board, the low-temperature co-fired ceramics, bulk silicon etching (micromachining), 3D printing, etc.